416 THE LIAS AMMONITES. 



whorls, which are about one-third involute. The aperture is oval or oblong, convex at 

 the sides, tapering towards the area, and slightly grooved by the return of the spire. 

 The sutural line is simple, and forms very symmetrical lobes and saddles. The siphonal 

 lobe, almost as long and wide as the principal lateral, is furnished on each side with 

 three points, and one branch with four. The siphonal saddle terminates in two unequal 

 folioles, of which the external consists of three festoons and the internal of two. Thfr 

 principal lateral lobe divides into two terminal branches, of which the external is the 

 larger. The lateral saddle terminates in one large external and two smaller internal 

 festoons ; the three other small auxiliary lobes decrease in size from without inwards. 



AJJimties and Differences. — The number of the large ribs which occupy the sulca- 

 tions varies from twelve to fourteen, they resemble the annular fimbriae seen on Lyt. 

 jimbriatum. The intermediate oblique annular ribs in Erench specimens, according to 

 d'Orbigny's figure, are in general extremely small, whilst in German forms, according to 

 Zieten, they are larger (PI. LXXVI fig. 6). When the shell was complete this 

 Ammonite must have had a very elegant form. When the shell is absent, as is 

 almost always the case in English specimens, the mould is smooth, with deep, wide, 

 smooth constrictions on the outer and first inner vi^horls. 



When the shell attains the diameter of seventy millimetres, according to d'Orbigny, it 

 loses its ribs and sulci entirely, and becomes altogether smooth ; the whorls then are very 

 large and always much inflated, and very unlike the inner whorls of the shell. 



This Ammonite, by its periodical interrupted constrictions in middle life, manifests 

 affinities with Lyt. jimbriatum and Lyt. cornucopia; it diflFers from both, however, in 

 having deep permanent sulcations on the mould, which are absent in these species ; the 

 suture-line likewise is much less complicated, and the whorls are compressed and never 

 cylindrical. A perfect specimen of this fossil would display one of the most beautiful 

 Ammonite structures with which I am acquainted, but the extreme thinness of its 

 delicate shell, with the small amount of involubility in the whorls, renders it one of the 

 Ammonite gems of the Lias formation. It resembles Lyt. torulosum (PI. LXXVI, fig. 5> 

 in general form, but differs from that species by its recurrent constrictions. 



Locality and StratiyrapJiical Position. — I have collected several fragments from the 

 Jurense-heA at Frocester Hill; and Prof. Blake reports a single fragment from Blea 

 Wyke, Yorkshire Coast, associated with Harpoceras striatulum. The specimen figured 

 PI. LXXVI, fig. 6, is from Germany. 



Foreign Distribution. — Schlotheim collected his numerous examples from the Jurensis- 

 Schicht at Aschach, north-east of Amberg, in the Oberpfalz ; Zieten's examples came 

 from Gross-Eislingen and Wasseralfingen ; it has been collected likewise from the same 

 zone at Altdorf, Heiningen, Uhrweiler, and Muhlhausen. D'Orbigny states that this 

 Ammonite was collected from the uppermost beds of the Upper Lias of the Charolles, 

 Saone-et-Loire ; in the environs of Salins, Jura ; at Villenotte, near Semur, Cote-d'Or ; 

 and at Verpilliere, Isere. 



